1. Field of Art
The present invention generally relates to measurement of nitrate-nitrogen concentrations in soil.
2. Description of the Related Art
Nutrient levels in soil have significant spatial and temporal variations. Accordingly, there has been significant effort placed into development of local nutrient management schemes, often referred to as “precision agriculture,” addressing nutrient level variation. Local nutrient management increases agricultural efficiency while reducing its environmental impact by allowing growers to locally apply nutrients where needed. Increases in nutrient costs and a growing awareness of the environmental consequences of current agriculture practices have made improvements in agricultural efficiency and environmental impact increasingly important.
Nitrate-nitrogen is one of most important nutrients for a variety of crops, but it is particularly mobile in the soil, making it subject to large spatial variations. The conventional approach to nitrate-nitrogen measurement is based on laboratory-based soil measurements. Soil samples are typically mailed to the labs, where the samples are unpacked, sorted, dried, ground, and then measured. This process is fairly expensive and can take up to two weeks before results are available. This can be a significant drawback.
As an example, in-season nitrogen management in corn-growing regions is often difficult because of the slow turnaround time of laboratory-based soil testing. Extending the time when corn growers are able to measure soil nitrogen levels would allow corn growers to test fields before their last application of fertilizer. This would enable corn growers to test fields later in the growing season and implement better nitrogen management practices. Further, allowing growers to promptly retest fields, such as retesting after a rain, would allow growers to adopt more efficient nitrogen management practices. Additionally, laboratory-based soil measurement costs scale directly with the number of samples, making it prohibitively expensive to sample at high grid densities.
As a result, there has been interest in developing faster, simpler and/or less expensive soil measurement techniques to expand the benefits of precision agriculture. Technologies used have ranged from mid-infrared (mid-IR) spectroscopy to ion-selective electrodes. However, each of these methods has suffered from some combination of expense, low accuracy, stringent calibration requirements or difficulty of use.
One approach is based on canopy sensors and satellite imagery that can measure NDVI (normalized difference vegetative index), which is essentially a color measurement that can be used to infer nitrogen needs. These methods are typically fast and operate on a relatively low cost/acre. Unfortunately, there are numerous interferences to NDVI measurements, as many factors can affect crop color, such as water needs and disease. Thus, it appears to suffer from low accuracy. Additionally, this method requires a dense crop canopy to be useful, which puts a tight operational limit on its use. It can only be used fairly late in the season.
There have also been several recent efforts to perform fast “on-the-go” measurements of soil nitrate-nitrogen using ion-selective electrodes. However, the fragility of the ion-selective membrane has caused significant problems with the robustness and reproducibility of soil measurements. Ion-selective systems also require frequent calibration, making them unappealing for routine field use.
Nitrate “strip tests,” commonly available from scientific supply stores or from manufacturers, have also been used. However, nitrate strip tests typically suffer from poor accuracy compared to standard laboratory-based tests and require extensive sample preparation, including consumable reagents. For example, the standard preparation time for nitrate strip tests typically approaches 30 minutes, includes numerous preparation steps and requires precise timing of the reaction steps.
In another recent approach, optical absorption has been used for in-situ monitoring of soil nitrate content. However, this approach was based on a filtering method, in which an optical probe was encapsulated inside a porous stainless steel casing. As a result, the method suffered from very slow measurement times (in the tens of hours). In addition, this approach was focused on measuring the nitrate absorption peak at 300 nm. However, the peak at 300 nm has a relatively weak absorption cross section, and so presents difficulties when measuring nitrate concentration values typically found in agricultural soils. For example, experimental results based on the 300 nm peak typically do not demonstrate sensitivity below 100 ppm nitrate-nitrogen concentration, whereas agronomically relevant levels of soil nitrate-nitrogen concentration are in the 0-50 ppm range.
Accordingly, a rapid and economical soil nitrate-nitrogen measurement system could significantly increase the efficiency of agricultural nitrate use.